In the road construction field, and especially for making pavements, the asphalt mixes are extensively known and defined as being aggregates coated with a hydrocarbon binder. The aggregates are traditionally a mixture of mineral fillers such as chips, sands and fines. The hydrocarbon binder is typically based on bitumen because since it advantageously changes its consistency depending on temperature, bitumen is particularly well suited to road construction: (1) at high temperatures, the bitumen decrease in viscosity enables to coat the mixture of mineral fillers during the production of asphalt mixes, then to compact said asphalt mixes during the implementation thereof; (2) while, at operating temperatures, the bitumen adhesive strength and cohesion enables to ensure stability to the mineral construction under traffic strain.
Bitumen rheological properties thus play a crucial role in the production of asphalt mixes, and in road construction and pavement performance, even if the bitumen content does generally not exceed 7% by weight of the asphalt mix total weight.
However, bitumen rheological properties are evolving, and performance in service of pavements mainly depends on such evolution.
On the one hand, pavements are submitted to the mechanical stresses of road traffic (which vary with the loading rate), and to thermal and oxidizing actions associated with the ambient conditions (air, water, UV, de-icing salts . . . ). Bitumen reacts to such mechanical and environmental stress factors by having three different types of behaviour with a specific pavement degradation:
(1) At high temperatures and/or at lower level of stress, bitumen has a viscous behaviour, which induces a plastic deformation of the bituminous mortar (the part made of sands and fines coated with bituminous binder contained in an asphalt mix) resulting in the pavement rutting if the fillers are not taken up by the mineral skeleton.
(2) At low temperatures and/or at higher level of stress, bitumen behaves as an elastic solid, which may lead, if vehicles do carry heavy loads, to a brittle fracture of the binder and to the formation of a crack appearing in the mortar; this is what is called the pavement thermal cracking.
(3) In the intermediate temperature range, the behaviour of bitumen is intermediate and said to be viscoelastic. When small deformations induced by the road traffic accumulate, then a gradual degradation of the bituminous mortar can be observed and finally cracks develop; this is what is called the pavement fatigue cracking.
On the other hand, bitumen properties change                As soon as production and implementation operations begin, bitumen as a thin film is carried to high temperatures in the presence of air and does undergo, in addition to a loss of the most volatile fractions, a substantial oxidation.        Such ageing phenomenon thereafter slowly continues within the existing asphalt during the whole life-cycle of the pavement; this is accompanied by a chemical change together with a hardening of the binder, which promotes the brittle fracture at low temperature (thermal cracking) and the development of fatigue cracking in the intermediate temperature range.        
As a consequence, the properties of bitumens and their evolution play an important role in the production, the construction and performance of pavements made with bituminous asphalt mixes. Bitumen is usually selected depending on the climate, on the level of traffic stress and on the final structural function of the asphalt (binder course, wearing course, sub-grade course . . . ). Because of the crucial influence of bitumen rheological properties in the production of asphalt mixes, and in the construction and performance of pavements, specifications have been established so as to classify bitumens according to their rheological properties, and to control bitumen rheological properties and their incidence onto the performance of pavements.
In Europe, the traditional most widely used specifications are based upon:                empiric tests relative to the consistency at room temperature (Penetration test at 25° C. according to the ASTM D5 or EN 1426 standard), and/or at higher temperatures (Softening test with ring and ball above 40° C., according to the ASTM D36 or EN 1427 standard), and/or        viscosity measurements so as to evaluate the pumping ability, the mixing ability and the compaction capacity (cinematic viscosity at 135° C. according to the ASTM D2170 or EN 12595 standard; rotating viscometer at 135° C. according to the AASHTO T-316 or EN 13302 standard) or so as to evaluate the risk of creep (dynamic viscosity at 60° C. according to the ASTM D D4190 or EN 12596 standard).        
Bitumens are thus classified depending on the results obtained in these tests. For example, a 20-30 grade bitumen means that such bitumen has a penetrability at 25° C. as measured according to the EN1426 standard, which varies from 20 1/10th to 30 1/10th of a mm.
However, the hereabove described traditional specifications do neither deal with the viscoelastic behaviour of bitumens, nor with their rheological properties at low temperature, nor with the impact of their ageing. That is why a program called Strategic Highway Research Program (SHRP) was developed in the United States, which gave rise to the so called “superpave” specifications (Superior Performing Asphalt Pavement). The Superpave specifications, which are described in the AASHTO M320-10 American standard, classify bitumens depending on their mechanical performances at high and low temperatures by integrating procedures, which simulate the ageing of the bitumen in the short and the long run.
The Superpave specifications thus define the performance grade of bitumen (PG) relative to a particular pair of temperatures, a maximum and a minimum one: the pavement highest temperature for the 7 hottest consecutive days, and the pavement lowest temperature for the coldest day. For example, a PG 58-28 means that the pavement highest temperature for the 7 hottest consecutive days is 58° C. and the pavement lowest temperature for the coldest day is −28° C. Moreover, the higher limit of temperature (for example 58° C.) does correspond to the temperature, at which the minimum resistance to creep has been reached, the creep being the physical phenomenon upon which the irreversible deformation of a material occurs, after having been submitted for a sufficient time to a constant stress that is lower than the elastic limit of the material. The lower limit of temperature (for example −28° C.) does correspond to the temperature at which the bitumen resistance to thermal shrinkage-induced stresses has been reached.
In the Superpave specifications, the bitumen mechanical performance criteria remain constant, only the temperatures do change, at which such criteria must be met. This enables therefore to select a bitumen depending on its mechanical performances for a climatic area and given road traffic conditions. Thus, bitumens with PG 64-22 and PG 52-34 are binders for which there is a risk of irreversible deformation above 64° C. and 52° C. respectively, and a risk of thermal cracking under −22° C. and −34° C. respectively. In North America, bitumens with PG 64-22 and PG 52-34 may respectively be selected for the Midwest and part of the Canada. The approach of the Superpave specifications is opposed to that of the traditional specifications described previously, the latter being based on properties which were determined at a given temperature and for which the mechanical performance criterion should be adapted to the climatic area.
Whatever the specifications used (either the traditional or the superpave), the choice of the bitumen grade for making an asphalt mix to be used in order to construct a pavement in a given geographical area as a rule is made by taking various parameters into account, and in particular depending on the climatic conditions of said geographical area, on the level of traffic stress onto said pavement, and on the final structural function of the asphalt mix (binder course, wearing course, sub-grade course . . . ). Thus, for given climatic conditions, for a given level of traffic stress, and a given structural function, there is one bitumen which grade enables to ensure optimum mechanical performances for the asphalt mix and therefore for the pavement; such bitumen is called “base bitumen”, or more generally speaking, “base binder”.
In practice, the parameter kept for selecting the grade of the binder used for making asphalt mixes is in particular the climatic conditions under which the asphalt mixes will be used. That is the reason why, in the road construction field, it is a common practice to make a base binder correspond to a given geographical area. In the American Southwest (for example, Los Angeles), the base binder usually has the PG 64-10. In the North-eastern quarter of the United States, the base binder usually has the PG 64-22. In Alaska, the base binder usually has the PG 52-34. In the South of France, the base binder is usually a binder with grade 30/50. In the East of France or in mountainous regions, the base binder usually has the grade 50/70.
As previously explained, hydrocarbon-based pavements do undergo an ageing phenomenon due to the climatic conditions and to traffic-induced stresses, such ageing expressing through the formation of ruts and/or fatigue cracks and/or thermal cracks. A rehabilitation of the pavement may then become necessary in order to preserve the road safety and to prevent any further degradation of the pavement body which would require a global reconstruction, and therefore would result in much more substantial rehabilitation expenses.
To rehabilitate a pavement, the first step usually consists in recovering recycled bituminous materials.
It is a common practice to use recycled bituminous materials originating from the recycling of damaged bituminous courses by lifting asphalt plates or by milling the same. The materials for reworking are then crushed/screened so as to have granulometrically homogeneous batches. The thus obtained recycled bituminous materials are called reclaimed asphalt pavements (RAP). Reusing RAP in bituminous asphalt mixes is a common practice in most European countries (EN 13108-8 and CEN 2005a standards) and in the United States where it has been incorporated in the method “standard specification for superpave volumetric mix design” (AASHTO M 323). RAP have indeed a mean composition similar to that of bituminous asphalt mixes classically produced and integrated within pavements, i.e. an average bitumen content by weight ranging from 3% to 6% relative to the RAP total weight, the reminder of the composition being a mineral mixture of chips, sands and fines. However, bitumen present in RAP did undergo an ageing process from the beginning of the production and construction steps, and thereafter gradually in the existing asphalt mix through oxidizing phenomena and chemical transformation. This ageing does express through the loss of volatile components, and therefore through hardening, which can be observed by simply measuring the RAP binder rheological properties, after extraction of the latter from the RAP. Thus, the binder of an RAP originating from a pavement located in a geographical area, for which the base binder is of PG 64-22 may correspond to a harder binder, for example to a binder with PG 82-16 or PG 88-10, or even with harder grades.
More recently, recycled asphalt shingles (RAS) originating either from damaged asphalt shingle batches, rejected as soon as they were made, either from asphalt shingles removed after roof repairs have also been employed as recycled bituminous materials. Once they have been cleared from any possible residues (wood, nails . . . ), recovered shingles are finely chipped and screened so as to facilitate their incorporation into asphalt mixes. The thus obtained recycled asphalt shingles generally comprise from 15 to 35% by weight of bitumen, from 50 to 60% of a fine mineral filler and from 1 to 12% of glass or cellulose fibers. The bitumen from RAS did undergo a stronger ageing phenomenon than reclaimed asphalt pavements; it is therefore typically harder than those used in road applications.
The second step of the pavement rehabilitation process consists in mixing and hot coating a given amount of RAP and/or RAS and of new aggregates with new bitumen (called “added binder”), and if necessary with other additives such as fibers and/or polymers. The amount of recycled bituminous materials (RAP and/or RAS) may vary from a few percents to almost 100% by weight relative to the final mixture total weight.
During this second step, the one or more binder(s) of RAP and/or RAS, called “aged binder(s)”, is or are activated upon contacting new overheated aggregates, under the hot air flow of the dryer, and is or are being mixed more or less completely to the new added bitumen (“added binder”). Rheological properties of the resulting binder (mixture of the one or more aged binder(s) given to the added binder), will depend on both the characteristics of the added binder, on those of the one or more aged binder(s), and on the proportion according which this or these aged binder(s) are comprised in the mixture formed with new bitumen; such proportion depending on both the RAP/RAS content in the asphalt, on their composition and on the activation rate of their binder. When the RAP/RAS content is low relative to the granular mixture total weight (new aggregates and recycled bituminous materials), incorporating the one or more aged binder(s) into the added binder will have a negligible effect on the grade of the added binder. On the contrary, when the RAP/RAS content is high, such as of from 30 to 65% and above, the effect of the one or more aged binder(s) on the rheological properties of the added binder is substantial.
The asphalt mixes obtained at the end of this second step will be used for constructing new pavements. Their mechanical properties will therefore have to ensure their workability and their compaction in a workmanlike manner, as well as the pavement performances in service. Now the hardening of the added binder through the one or more aged binder(s) of recycled bituminous materials may lead to workability problems for the asphalt mix and to higher risks of cracking in the pavement.
It is therefore necessary to adapt the grade of the added binder so as to compensate the stiffening of said binder, as a result of the combination thereof with the one or more aged binder(s) of the recycled bituminous materials. The SHRP program plans 3 cases:
Case no 1) For making an asphalt mix comprising a low content of recycled bituminous materials, for example lower than 15% by weight relative to the granular mixture total weight (new aggregates and recycled bituminous materials), that is to say of from 0 to 17% by weight of the aged binder(s) relative to the total weight of the mixture of added binder and aged binder(s), adding the one or more aged binder(s) to the added binder will have a negligible effect on the grade of the added binder. As a consequence, no adaptation of the added binder is required; the added binder usually has the same grade as the base binder of the pavement for which the asphalt mix is intended to be used.
Case no 2) For making an asphalt mix comprising an intermediate content of recycled bituminous materials, for example of from 15% to 30% relative to the granular mixture total weight (new aggregates and recycled bituminous materials), that is to say of from 18% to 27% by weight of the aged binder(s) relative to the total weight of the mixture of added binder and aged binder(s), the added binder is selected with a softer grade than the base binder of the pavement for which the asphalt mix is intended to be used. Thus, if the base binder is of PG 64-22, the added binder is for examples selected from the PG 58-28 and PG 52-28 grades.
Case no 3) For making an asphalt mix comprising a high content of recycled bituminous materials, for example of more than 30% by weight relative to the granular mixture total weight (new aggregates and recycled bituminous materials), that is to say of more than 28% by weight of the aged binder(s) relative to the total weight of the mixture of added binder and aged binder(s), the grade of the added binder will be determined in such a way that the binder resulting from the mixture of aged binder and added binder meets the performance criteria of the SHRP characterization of the geographical area. The added binder is therefore necessarily chosen with a softer grade than the base binder of the pavement for which the asphalt mix is intended to be used.
Using an added binder with a softer grade than that of the base binder has many drawbacks: (i) binders with soft grades are commonly more expensive than those with harder grades; (ii) multiplying binders in the coating plant is problematic and results in additional costs for logistics (deliveries, storage . . . ) because a tank must be necessarily provided for each binder grade.